The present invention relates to an ISDN telecommunication system as defined in the preamble of claim 1 and a method for implementing an ISDN PBX interface as defined in the preamble of claim 8.
In the ISDN system (ISDN, Integrated Services Digital Network), two access types have been defined: base access (BA) and primary rate access (PRA). Via a base access, it is possible to connect to the public telephone network an individual terminal device or a set of max. 8 terminal devices, two of which may be active at the same time. The channel structure of a base access is 2B+D, where B is 64 kbit/s information channel and D is a 16 kbit/s signalling channel. In a primary rate access, the corresponding structure is 30B+D, where D is a 64 kbit/s signalling channel.
The primary race access is designed to connect an ISDN PBX (Private Branch Exchange), which typically is a private branch exchange used by a company or other association, to a telephone exchange. If not all of the thirty access lines in the ISDN PBX are needed, then the extra channels in the primary rate access remain unused. Still, the user of the ISDN PBX must bear the full cost of a 30B+D-type primary rate access. A known solution is to have a plurality of 2B+D base access lines connected to the ISDN PBX. In this case, instead of a single 2048 kbit/s primary rate access PCM cable (PCM, Pulse Code Modulation), a plurality of cables and extra equipment are needed and not all of the services associated with a primary rate access are available to the user.
The object of the present invention is to eliminate the drawbacks described above. A specific object of the present invention is to disclose a new type of system in an ISDN network, designed to enable a plurality of branch exchange interfaces to be connected to a single primary rate interface.
As for the features characteristic of the invention, reference is made to the claims.
The invention concerns a telecommunication system which comprises a telephone exchange provided with ISDN properties, including an information channel B and a signalling channel D. Connected to the telephone exchange is a primary rate interface. An ISDN PBX is connected to the telephone exchange via a PBX interface similar to a primary rate interface. The system of the invention comprises means for dividing a primary rate interface into at least two PBX interfaces.
In a preferred embodiment of the invention, the system comprises means for converting a B channel in the primary rate interface into a D channel, thus changing the 30B+D type primary rate interface into a (31xe2x88x92n)B+(n*D) type interface, n being an integer greater than zero.
In a preferred embodiment of the invention, the system comprises a multi-channelling device connected to the primary rate interface and to the PBX interface. The system preferably comprises means for removing the D channel converted from a B channel into time slot no. 16 of the PBX interface.
In a preferred embodiment of the invention, the system comprises means for distributing the messages transmitted in the D channel of the primary rate interface into corresponding D channels in the PBX interface on the basis of the information (e.g. Channel identification and Call reference) carried by the messages.
In the method of the invention, the primary rate interface is divided into at least two PBX interfaces. In an embodiment of the method, one of the B channels in the primary rate interface is converted into a D channel, the converted D channel being preferably transmitted in time slot no. 16 of the PBX interface. The B channel is preferably transmitted in the same channel in both the PBX interface and the primary rate interface.
In an embodiment of the method of the invention, the information transmitted in the D channel of the primary rate interface is distributed into the D channels of the PBX interfaces.
As compared with prior art, the invention has the advantage that it provides an ISDN PBX interface with a lower capacity and at lower costs than a primary rate interface for small companies and equivalent associations. The exchange can be connected via a single PCM cable to means dividing a primary rate interface, said means being further connected via cables to two or more separate PBX interfaces. Thus, the costs of a primary rate interface can be distributed among several users, who still receive the subscriber services provided by a primary rate interface. From the operator""s point of view, the invention allows separate billing of each user.